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    \bfseries \zihao{3} Abstract
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\par What is a neural network and how does it successfully work? 
This article provides an introduction of neural networks, an
overview of the optimization algorithms for training and some
main theory questions regarding Deep Neural Networks. First, we
start with some basic intuitions of a neural network and some detailed
implementations about forward propagation and backward propagation. 
Second, we review the obstacle in training Deep Neural Networks and 
the corresponding solution including initialization and generic optimization
algorithms. Third, we discuss the three main theory questions about 
Deep Neural Networks.

